1. Field of the Invention
The present invention relates to a leak detecting apparatus using a compound turbo-molecular pump. The apparatus is used for detecting a gas leakage of equipment requiring gastightness such as a compressor, a condenser to be used in a refrigerator or the like. In the present invention, the term "compound turbo-molecular pump" means a pump which is defined as a combination of a turbo molecular pump and a drag pump (or a turbo molecular pump or a screw pump in place of the drag pump).
2. Description of Related Art
Conventionally, the leakage of a gas in an object of inspection (or an object to be inspected) has been detected in the following manner. Namely, the object of inspection such as a compressor or the like is placed in a helium gas atmosphere or helium gas is blown onto it. The inside of the object of inspection is connected to a mass analyzing (or mass spectrometer) type of leak detecting apparatus as shown in FIGS. 1 and 2. Helium gas leaking from a leaking point into the inside of the object of inspection is thereby detected. In the leak detecting apparatus shown in FIG. 1, which is of a direct introduction type, when there is a leakage in the object "a" of inspection, the helium gas leaked into the object "a" of inspection is discharged outside from a test port b via a gate valve (or a shutoff valve) c, an analyzer tube d, a turbo molecular pump e, and an auxiliary vacuum pump f, so that the helium gas is detected in the analyzer tube d. In this apparatus, since the analyzer tube d is located within the discharge passage through which the helium gas introduced into the test port b is discharged, a high detecting sensitivity can be obtained and the reaction speed to the helium gas is quick.
On the other hand, the leak detecting apparatus shown in FIG. 2, which is of a back-diffusion type, utilizes a molecular movement of the helium gas. The helium gas leaked into the inside of the object "a" of inspection is discharged from a test port b into a roughening vacuum valve g and an auxiliary vacuum pump f. On its way, the gas is partly back-diffused to an analyzer tube d via a turbo molecular pump e, and is mass-analyzed in the analyzer tube d (or mass spectrometry takes place in the analyzer tube d). The above-described back-diffusion is a phenomenon which occurs when the compression ratio of the turbo molecular pump itself is small relative to a small, light molecule such as of helium gas. In this apparatus, since the measurement is possible if the exhausting or evacuating can be made down to an internal pressure of the object "a" of inspection of about 100 Pa, there is an advantage in that the time for rough-exhausting before starting the leak detection may be short.
In the apparatus of direct introduction type as shown in FIG. 1, the pressure (10.sup.-3 Pa) inside the analyzer tube d required for the mass spectrometry becomes equal to the internal pressure of the object "a" of inspection. Therefore, it takes much time before the pressure inside the object "a" of inspection attains a value that fits for inspection at d. It is therefore not suitable for the leak detection of an object of a large volume. On the other hand, the apparatus of back-diffusion type as shown in FIG. 2 requires a shorter time in starting the leak detection. It has, however, a disadvantage in that, due to the characteristics in the exhausting or evacuating system, the detecting sensitivity and the reaction speed are inferior to those of the apparatus of direct introduction type. These conventional apparatuses thus have both advantages and disadvantages and it is left to the experience of an operator to choose which of the direct introduction type and the back-diffusion type systems is to be used for the leak detection. It follows that an adequate selection cannot be made by an operator without an experience in the problems normally encountered in the leak detection of various objects of inspection.
In view of the above disadvantages, the present invention has an object of providing a leak detecting apparatus which can start the leak detection of an object of inspection in a shorter time and which is superior in the detecting sensitivity and the reaction speed and is easier to operate.